Photodynamic therapy (PDT) is a treatment modality using light of an appropriate wavelength to activate a photosensitizer in the presence of oxygen, which generates active oxygen species of high reactivity then the target molecule thereby leading to the tissue damage. Owing to its advantages such as relative selectivity in most sites, its compatibility with other treatment, its repeatability, its ease of delivery etc., PDT is slowly finding its place as a useful method for the treatment of certain cancers or clinical situation, such as early stage cancer of the lungs, esophagus, stomach, cervix, cervical dysplasia etc. Prime features of the ideal photosensitizer are low dark toxicity, selective accumulation in malignant cells, appropriate retention time, absorption in the phototherapeutic window (600-900 nm) and high triplet yield with long time to decay. Various photosensitizers have been used for PDT, the first generation of the photosensitizers is based on porphyrin structure, Photofrin II®, for disseminated i.p. malignancies have received most of the attention. Limitation of a currently used photosensitizer, Photofrin-II®, which include prolonged cutaneous photosensitivity, batch variability, difficulty with purification, monomeric form as complicated serum, normal tissue and the tumor pharmacokinetics. The poor light absorption in the therapeutic window is suboptimal in terms of light penetration in tissues. These undesired features have prompted the development of second generation of photosensitizers more amenable to site directed chemical modification to improve physicochemical, pharmacological and clinical properties (Miller et al. Drug Devel. Res., 42, 1997, 182).
Lown and co-workers (Lown J. W. et al. Tetrahedron, 48, 1992, 45), (Lown J. W. et al. Photochem. Photobiol., 52, 1990, 609) have addressed this problem associated with Photofrin-II® by modifying perylenequinonoid pigments (PQP), which are derived from the natural sources (especially from fungus) exhibits intriguing stereo chemical features and possess interesting biological activities. Of these, hypocrellin A (compound (1)) and hypocrellin B (compound 2)), which are lipid-soluble perylenequinone derivatives (Chen et al. Liebigs Ann. Chem., 1981, 1880) (Kishi et al. Planta Med., 57, 1991, 376) isolated from the fungus Hypocrella bambuase sacc a parasitic fungus of the Sinarundinaria species growing abundantly in the southern China in the region of Yunnan Provience, southeastern region of Tibet and certain parts of Sri Lanka, have served as the starting point for the development of new improved photosensitizers.

Hypocrellin A and B have been intensively investigated because of their light induced anti tumor (Zhang et al. Photochem Photobiol, 69(5), 1999, 582), (Zhang et al. J. Photochem Photobiol. 44, 1998, 21) and antiviral activity (Hirayama et al. Photochem. Photobiol. 66(5), 1997, 697) termed, technically known as Photodynamic therapy (PDT). Hypocrellins were first recognized as potential photosensitizers for PDT (Wan et al. Kexue Tongbao (English Edition), 26, 1981, 1040-1042) in the early 1980s. Hypocrellins are efficient singlet oxygen generators during photochemical reactions and may also exert photosensitization via radical mechanisms, which may confer a degree of independence from classical type II oxygen dependent photochemical mechanism. Preliminary acute and chronic dose escalation studies of hypocrellins and their derivatives have failed to demonstrate any toxic properties in rodents to “total-body” levels of 50 m mol/Kg or approximately two logs higher than typical in vitro photosensitizing dose. Hypocrellins have several advantages over the other photosensitizers like easy preparation and purification, low toxicity, high stability, no aggregation, rapid metabolism, low side effects and selective localization in cancer tissues. However the natural occurring compounds are only lipid soluble and exhibit little absorption in the photodynamic window which limits their application in PDT. In order to overcome these issues, i.e. lack of photodynamic activity and water solubility, a large number of hypocrellin-based compounds have been synthesized and biologically evaluated in the last twenty years.
Therefore, various approaches have been adopted to increase the red absorption of the hypocrellin B (Shangjie et al. Photochem. and Photobiol., 78(4), 2003, 411), (US Patent application published under No. US2004/0092557 A1).
However, there is still a clear need for improvement. It would thus be highly desirable to be provided with a compound having an enhanced photodynamic activity as compared to hypocrellin A and hypocrellin B.